The signals transmitted in the orthogonal frequency-division multiplexing mode will hereinafter be identified by the short name of OFDM (Orthogonal Frequency-Division Multiplexing). The OFDM technique consists of splitting up a transmit information signal by spreading it over a large number of digital low-rate channels. Thus a strongly selective wideband channel is transformed into a large number of non-selective digital channels. As they all form one wideband channel, it is not very likely that fadings simultaneously affect the whole channel in the course of the transmission. This technique also permits of reducing intersymbol interference. Thus, one frequency corresponds with each digital channel, while all the frequencies are symmetrically spread around one central carrier frequency. As it is hardly acceptable to use selective filters at the receiver end, it is preferred to tolerate an overlap of the spectrums, but while imposing conditions of orthogonality between the frequencies to eliminate the intersymbol interference at the sampling instants.
The OFDM signal is organized in frames formed by blocks of which certain blocks are service blocks and others are data blocks. To avoid intersymbol interference, each block contains redundancy data. A random block is formed by all the OFDM frequencies used by the system, the frequencies being modulated by digital transmit symbols caused by a coded modulation, for example, a digital PSK or QAM modulation. At the transmitter end, these symbols areas coded in accordance with a certain format and with a certain timing which is to be detected at the receiver end so that they can be decoded correctly. Thus, at the receiver end, the received signals are to be demodulated and then decoded to restore the original data. The invention proposes to retrieve the essential characteristic features of the transmission format used for the transmission.
European patent EP 608 024 which corresponds to U.S. Pat. No. 5,559,833, issued Sept. 24, 1996, assigned to U.S. Philips Corporation, describes a digital transmission system which has a timing recovery of OFDM signals. This document exploits the two occurrences of a like group of symbols which appear in an OFDM signal. A correlation is established between these two occurrences by delaying one of the occurrences by a constant delay. Indeed, this document considers this delay to be known, which is not the case in all the applications of this technique. The device thus described recovers the timing of the symbols on the basis of a local clock, on a frequency division and on a control by a phase-locked loop. Moreover, the correlation utilizes a technique of subtraction which necessitates the use of much memory capacity.